Various known devices use speckle images and correlation between speckle images to determine deformations and/or displacements. A speckle image is generated by illuminating an optically rough surface with a light source. Generally, the light source is a coherent light source, and more particularly, is a laser-generating light source, such as a laser, a laser diode, and the like. After the optically rough surface is illuminated by the light source, the light scattered from the optically rough surface is imaged onto an optical sensor, such as a charge-coupled device (CCD), a semi-conductor image sensor array, such as a CMOS image sensor array, or the like.
Prior to displacement or deformation of the optically rough surface, a first speckle image is captured and stored. Subsequently, after deformation or displacement of the optically rough surface, a second speckle image is captured and stored. The previous and subsequent speckle images are then compared on a pixel by pixel basis. In particular, a plurality of comparisons are performed. In each comparison, the previous and subsequent speckle images are offset relative to each other, and the amount of offset is increased by one image element, or pixel, between each comparison. In each comparison, the image value of a particular pixel in the previous image is multiplied by, or subtracted from, the corresponding subsequent image pixel (based on the amount of offset) and the result is accumulated for all pixels to determine a correlation value. The offset having the best correlation value between the subsequent and previous images will generate a peak or a trough when the set of correlation values is plotted against the offset.
For example, U.S. Pat. No. 4,794,384 discloses a mouse which uses speckle-image-correlation to determine two dimensional motion directions of a mouse. However, in the '384 patent, the speckle-image-correlation does not need to be performed at a high rate and the accuracy only needs to be on the millimeter range.
U.S. Pat. No. 4,967,093 discloses systems and methods for measuring deformation of an object using speckle-image-correlation. In particular, the '093 patent describes in detail various conventional methods for comparing two speckle images and for determining when to update a current reference speckle image with a new reference speckle image. Similarly, published Japanese Patent Application 8-271231, published October 1996, discloses additional methods for avoiding accumulating error in a speckle-image-based displacement gauge.
Japanese Patent Application No. 5-52517, published March 1993, discloses a speckled-image-based displacement meter that uses a rectangular or elliptically shaped slit 51 in a slit plate 5. The light beam from the laser light source passes through the slit 51 before it illuminates the optically rough surface. Thus, the light beam is shaped by the slit 51. The shaped light beam allows an amount of displacement in a main displacement direction to be measured with high sensitivity while displacement components in a perpendicular direction relative to the main displacement direction tends not to affect the measurement.